Guidelines for Left-Turn Phasing Treatment

The safety of left-turning vehicles at signalized intersections is a concern of the safety community, and guidelines for the installation of left-turn phasing have been developed. The development of guidelines to be considered when determining whether left-turn phasing should be used and the type of phasing that would be appropriate is presented. The guidelines are based on accident data collected for 408 approaches in Kentucky and delay data simulated for a variety of intersection characteristics. The recommendations made take into account many variables, including left-turn volumes, accident rates, product of opposing and left-turn volumes, and left-turn delays. A two-step approach is considered in the decision whether to install left-turn phasing: determine whether protection is needed and then determine the type of protection. The selection of the appropriate left-turn treatment is a process in which trade-offs between safety and delays may be required. Moreover, the use of single rules is usually not adequate to determine the appropriate left-turn treatment; a variety of other factors must be considered.